Method for the forming of ceramic green parts

ABSTRACT

The well known slip casting process for the production of ceramic green parts, in which hardening is achieved by water removal with porous molds, is characterized by disadvantages in terms of strength, shrinkage during drying and problems with cracking during drying. The present invention avoids these disadvantages and produces ceramic green parts by changing the surface potential of powder particles in the slip instead of by removing water. Further more the coagulation strength is increased by adding polymers or extremely fine divided colloidal particles.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation-In-Part of U.S. Ser. No. 08/211,139,filed May 9, 1994 now abandoned.

BACKGROUND OF THE INVENTION

The present invention is directed to a process for the production ofceramic green parts.

The following five forming methods are normally used to produce ceramicgreen parts:

1) pressing

2) isopressing

3) extrusion

4) slip casting and

5) injection molding.

The selection of the forming method is based on the shape, function, andnumber of pieces of the required part.

Free flowing, easily pressed starting powders are required for pressingand isopressing. For extrusion of parts with axial symmetry, powdermixtures with organic or inorganic plastifiers are used. As in polymerprocessing, injection molding is needed for complex shaped green parts.This process uses a mixture of a ceramic powder and a thermoplast whichis put into a mold, and heated after forming to burn off thethermoplastic additives.

In slip casting, the ceramic powder and water are mixed withdeflocculants and binders to give a free flowing slip, which is thenpoured into porous molds. The extraction of water by the mold (i.e.molds of gypsum or porous plastics) leads to the formation of greenparts, which are dried after demolding and are subsequently sintered.This well known process is used to produce hollow green parts. Althoughuseful for a variety of applications, it has several disadvantages:

1) the hardening of the green body depends upon the extraction of water;Therefore migration of soluble slip components may occur, resulting inan unequal particle distribution in the shaped green part,

2) the forming process is slow, and

3) density gradients are observed in the green body.

In addition, the final products possess low green strength and the greenbody workability is limited. Careful and time consuming drying of theporous mold is necessary before reuse. In addition, the casting ofcomplex shapes is difficult and undercuts are hard to achieve.

Several attempts have been made in recent years to improve ceramicprocessing by transforming homogenous powder suspensions into solid likegreen bodies. This can be achieved by either consolidating thedispersion medium or by flocculating or coagulating the suspensionparticles.

Consolidating the dispersion medium is achieved in gel-casting whenmonomers in the suspension are polymerized. Polymerization can beinduced by UV radiation, applying heat or by catalysts. In freezingcasting the dispersion medium is consolidated by freezing and removedafter demolding by sublimation.

Flocculation and coagulation of a highly dense packed particle systemuses the control of interparticle forces in order to accomplish aliquid-solid transition. Flocculation and coagulation may by inducedeither by applying heat, or by changing the ionic strength of thesuspension. A steric stabilized poser particle suspension has been usedwhich becomes destabilized upon heating to form a rigid green body. Onemay first coagulate a suspension by adding salt, then increase thesolids loading of this destabilized suspension to form a coagulated,plastic behaving, clay like mass which, after destabilization, fills inthe mold. One may also work along the same line to produce a coagulated,clay like mass which, after destabilization can be vibrated into a moldto form the green body which is then demolded. The process is timeconsuming and produces unreliable ceramic parts. In this process thepowder particle network which is formed during coagulation is disturbedduring the shaping process by deforming the clay like mass into themold.

SUMMARY OF THE INVENTION

The present invention has as its object an improved slip casting processwhich eliminates these disadvantages.

Pursuant to the invention this object is solved and there is provided aprocess for the production of ceramic green parts from a castable,aqueous slip wherein an active substance is added to the slip whichchanges the surface of the slip particles leading to solidification ofthe slip.

The invention provides a technically simple, quick, and inexpensivealternative to other known forming methods. In contrast to conventionalslip casting, improvements in forming complex shaped green parts arepossible, while the required microstructure, surface properties andmechanical properties are maintained. For example, this new slip castingprocess very largely suppresses migration and heterogeneous distributionof soluble components as the hardening step is not dominated by waterwithdrawal. In addition, the development of density gradients areavoided as the forming process is faster than in normal slip casting.

BRIEF DESCRIPTION OF THE DRAWING

The drawing shows a flow chart of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The new process is based on the idea that oxidic or non oxidic particleswith an oxide surface layer react differently with water depending uponthe pH (potential determining ions are H⁺ and OH⁻). The reactionproduces a surface charge on the powder particles, creating a positivecharge in acidic-water, and a negative charge in alkaline water. Theresulting electrostatic charge (potential) determines the viscosity ofthe slip. In general, the higher the absolute value of the potential,the lower the viscosity and thus the better the castability of the slip.In between the alkaline and acidic region is the isoelectric point orIEP. At this point, the particles remain uncharged and the surfacepotential is almost zero. The viscosity of the slip is thus very highand it is no longer castable, i.e. it is rigid. The present inventiontakes advantage of this phenomenon to solidify the slip after casting byshifting the pH of the suspension to the IEP. This is in contrast toconventional, state of the art slip casting technology, where the greenbody is formed by water removal.

It is also in contrast to the practice that adds salt to the slip beforeshaping the ceramic green body part in order to produce a clay like masswhich after coagulation is vibrated or centrifuged in the mold which isthe shaping step of the green part. In accordance with the presentinvention, a stable, deagglomerated ceramic particle suspension is madeby mixing powder particles with water and acid or base in order toobtain a charge powder particle surface at a pH of the slip which is faraway of the isoelectric point of the powder. Then one (a substrate) ortwo substances (a substrate and a catalyst) are dissolved in this slip.These substances do not change the pH of the slip immediately. Then thestable slip is degassed in order to remove all air bubbles from theslip. Then the slip is cast into a mold which does not extract waterfrom the slip like in conventional slip casting. After casting the timedelayed, temperature sensitive decomposition reaction and/or the enzymecatalyzed decomposition reaction of the substrate produces productswhich shift the pH of the suspension towards the isoelectric point ofthe ceramic slip. Thereby the slip is destabilized(coagulated/flocculated) and solidifies to a solid, wet ceramic greenbody. Then the body can be handled and can be taken out of the mold,dried and sintered. A flow chart of the present invention is shown inthe drawing.

As the time delayed reaction is thermally activated it can be controlledby controlling the temperature of the slip. By cooling the slip and/orthe mixture of the slip and the catalyst to low temperatures, justslightly above the freezing temperature of the slip, the reaction can beconsiderably delayed compared to room temperature. By warming up totemperatures around 30° to typically 40° C., the reaction can beaccelerated.

The preferred type of acid or base (substrate) is an agent which willdecompose in the slip itself due to time delayed reactions. Thedecomposition products will then cause a shift in the pH of thesuspension to the IEP, so producing solidification. This change in pH ispossible starting from either the acidic or alkaline regions. Thepresent invention favors use of organic molecules as the active agents(substrate).

In accordance with the present invention, suitable active substrates areorganic molecules such as urea, carboxylic acid esters, e.g. acetic acidesters, decomposable carbohydrates, esters of glycerine, or carboxylicacid amides, used separately or as a mixture. If a mixture of an enzymeand one of the above mentioned substrates is used (the enzyme andsubstrate could also be added separately to achieve the solidificationof the slip), hydrolases (e.g. urease, carboxylesterase, pectinesterase,acylase and lipase or oxydases such as glucose oxydase) are preferred.The role of the enzyme is to decompose the active substrate dependingupon its nature to an acid or base and a second molecule. The acid orbase is therefore a reaction product formed in the slip itself.

If a time delayed, temperature sensitive decomposition of the activesubstrate is preferred, i.e. without the influence of enzymes, thepresent invention uses as preferred substrates glycerolesters such asglycerinetriacatate, glycerinediacetate, or lactones such as gluconicacid lactone or glucuronic acid lactone, or sultones, as1,3-propane-sultone or 1,4-butone-sultone.

The above mentioned pH shift is also used to increase the coagulationstrength of the green body and to decrease the coagulation duration(solidification time). The pH sensitive network reaction of polymermolecules or the ion sensitive network forming of other polymers iscontrolled by this pH shift. Specially modified polyacrylate emulsionsdo not exhibit a change in viscosity at a pH of 4, while they show anincrease in viscosity during a pH alteration to about 8 to 9. Thecombination of both, the solidification reaction and the pH sensitivedischarge of an acidic ceramic slurry (caused by a urease catalyzeddecomposition of urea), leads to an increase in coagulation strength bya factor of 5 to 10 which may be understood as a synergy effect.

In addition to the high coagulation strength, the green body may bedemolded much easier in the wet condition (not dried). The same effectmay be accomplished from the alkaline region. By using special sols(e.g. SiO₂ sol, an ultra-fine colloidal suspension with particles from 1to 20 nm Ludox TM, DuPont!) the coagulation strength is increased by thecoagulation of the sol due to an increase of the ionic strength. For thesame purpose, biopolymer based colloidal systems may be used. Theviscosity of such solutions, e.g. carrageenan solution, may be increaseddue to time-delayed pH changing reactions by adding ions (e.g. ammoniaor calcia ions), where the primary function of the ions is to cross-linkfunctional groups (e.g. carboxylic groups) of the polymer molecules, andnot predominantly to change the charge state of the powder particles.

A further increase in coagulation strength may be achieved by the use ofpolymers with ester groups. These polymers exhibit low viscosity in theesterified form. The formerly inactive carboxylic groups may showintermolecular interaction by hydrolysis of these ester groups. Theresulting bridge forming between several macromolecules leads to adensification of the binder.

The above described ester hydrolysis may either take place in a strongalkaline environment by basic catalysis or in a weak alkalineenvironment by using enzymes which hydrolyse esters (Esterases).

The coagulation duration may be lowered by a factor of 2 to 4 by atemperature increase of 10° to 20° C.

Also this invention proposes an alternative means of moving to the IEPwhich involves moving the whole zeta-potential curve (specific for eachcomposition) either from the acidic or basic region along thesolid-specific (e.g. aluminum oxide) resident zeta-potential curve. Thisis made possible by addition of acids into the acidic region and basesinto the basic region. In other words, the zeta-potential curve is movedfrom any specific point in the direction of the IEP (which is situatedon the abcissa of the zeta-potential diagram) and the liquid slip isthen solidified. The movement of a zeta-potential curve from a basicinto an acidic region or vice versa is chosen, so that slips which arehighly loaded with solid can nevertheless be cast. They would otherwisebe too stiff. For solidification of the now castable slip, an oxidizingreagent is added, which is able to decompose the deflocculant,internally. As a deflocculant for shifting the IEP to the acidicregions, 2,3,4-trihydroxybenzoic acid is preferred. As an oxidant forthis deflocculant, hydrogen peroxide has been shown to be useful.

Another means of shifting the zeta-potential curve from the alkaline tothe acidic region is by the addition of4,5-dihydroxy-1,3-benzenedisulfonic acid as an organic deflocculant, andby use of a deflocculant decomposing enzyme, e.g. catechol oxidase. Toshift the zeta-potential of a casting slip from the acidic to thealkaline region, in accordance with the present invention, organicmolecules are added which, when coupled by an enzyme, become activedeflocculants. These molecules are neither derivatives ofacetylsalicyclic acid or precursors of citric or maleic acid. Aspreferred enzymes, arylester hydrolase or citrate synthase are used.

The invention is further illustrated by the following examples but it isnot intended that these examples in any way be taken as limiting infield of application.

EXAMPLE 1

An acidic casting slip was prepared by adding 536 g aluminum oxidepowder (HPA 0.5 with 0.05 wt. % MgO; Ceralox Corporation, USA) to asolution of 1.4 g conc. hydrochloric acid and 3.0 g urea in 97.0 g ofdeionized water. After ultrasonification the deagglomerated slip had atypical viscosity of 100 to 300 mPa s.

500-1000 units urease (EC 3.5.1.5) were added as a solution of 2000units/ml urease to this processed slip to shift the pH along thezeta-potential curve towards the IEP (to the alkaline region). Afterhomogenization of the ready-to-cast slip, it was poured into polymer ormetal molds. Depending upon the reaction conditions, solidification ofthe slip occurred after 0.5 to 3 hours. Additional drying increased thestrength of the green bodies. Demolding was done a few hours aftercasting either in the wet or in the dried condition. Increasing thetemperature by 10° C. will reduce coagulation time by a factor of two.

The dried green parts were characterized by high green densities (58 to62 vol. % depending upon the solid content of the casting slip) and byexcellent sinterability.

EXAMPLE 2

An alkaline casting slip was prepared by adding 425 g aluminum oxidepowder (HPA 0.5; Ceralox Corporation, USA) to a mixture with 0.85 gcitric acid, 4.5 g acetic acid ethylester, 1.0 ml conc. ammonia solution(25%) and 73.0 g deionized water. The IEP of this slip was between 4 and4.5. After deagglomeration and degassing a typical viscosity of 50-200mPa s and a pH of 9 to 10 was observed for this castable slip.

To this slip, 100-200 units esterase (EC 3.1.1.1) were added as anaqueous solution. After casting, the enzyme addition lead to a pH shiftfrom the alkaline region to the acidic region along the zeta-potentialcurve up to the IEP. After casting into metal or polymer moldssolidification occurred depending upon reaction conditions in 1 to 3hours.

The dried green parts were characterized by high green densities (60 to62 vol. % depending upon solid content of the casting slip) and byexcellent sinterability.

EXAMPLE 3

An alkaline casting slip with a pH of 9 to 9.5 was prepared by adding425 g silicon carbide powder (Norton 10 LXC, Norton, Norway) to amixture of 0.5 g conc. ammonia solution (25%), 3.5 g glucose and 100.0 gdeionized water. After deagglomeration and degassing a typical viscosityof 500-1000 mPa s and a pH of 9 to 10 was observed for this castableslip.

To this slip 2000 units glucose oxidase (EC 1.1.3.4) were added as anaqueous solution with 1500 units/ml enzyme. After casting into metal orpolymer molds solidification occurred depending upon reaction conditionsand due to the shift of the suspension pH from the alkaline regiontowards the IEP in 0.5 to 2 hours. The dried green parts werecharacterized by green densities of 58 to 60%.

EXAMPLE 4

An alkaline casting slip with a ph of 10 was prepared by adding 260 galuminum oxide powder (HPA 0.5; Ceralox Corporation, USA) to a mixturewith 0.20 g Tiron (4,5-dihydroxy-1,3-benzenedisulfonic acid) and 0.5 gconc. ammonia solution (25%) in 49.0 g deionized water. Afterdeagglomeration and degassing 1.0 g glycerine triacetate was added undervacuum to the high viscosity slip (typical viscosity 50-200 mPa s).After casting into metal or polymer molds solidification occurred due tothe shift of the pH from the alkaline to the acidic region in 15-30minutes.

Green densities of 58 to 60% were reached.

EXAMPLE 5

An alkaline casting slip with a pH of 10-11 was prepared by adding 260 galuminum oxide powder (HPA 0.5; Ceralox Corporation, USA) to a solutionof 0.20 g 2,3,4-trihydroxybenzoic acid and 0.30 g conc. ammonia solution(25%) in 50.0 g deionized water. After deagglomeration and degassing0.50 ml hydrogen peroxide (30%) was added to the slip (viscosity ca. 500mPa s). After casting into metal or polymer molds solidificationoccurred due to the shift of the IEP from the neutral to the alkalineregion and by simultaneous shifting of the slip pH along the now changedzeta-potential curve to the IEP in 1 to 2 hours. The green parts werecharacterized by an extremely low content of organic additives (lessthan 0.1 wt. %) and had green densities of 58 to 60%.

EXAMPLE 6

An alkaline casting slip with a pH of 9-10 was prepared by adding 260 galuminum oxide powder (HPA 0.5; Ceralox Corporation, USA) to a solutionof 0.60 g Tiron (4,5-dihydroxy-1,3-benzenedisulfonic acid) and 1.30 gconc. ammonia solution (25%) in 50.0 g deionized water. Afterdeagglomeration and degassing 500 units catechol oxidase (EC 1.14.18.1)as a solution with 100 units/ml were added to the low viscous slip(viscosity 100-500 mPa s). After casting into metal or polymer molds,solidification occurred due to the shift of the zeta-potential curve.This was caused by the decomposition of the added deflocculant (Tiron)due to its reaction with the added enzyme catechol oxidase. The greenparts were characterized by a low content of organic additives (lessthan 0.5 wt. %) and had green densities of 58 to 60%.

EXAMPLE 7

An acidic casting slip with a pH of 5-6 was prepared by adding 280 galuminum oxide powder (HPA 0.5; Ceralox Corporation, USA) to a solutionof 0.60 g 2-carboxyacetylsalicylic acid in 50.0 g deionized water. Afterdeagglomeration and degassing, 500 units arylester hydrolase (EC3.1.1.2) in 0.5 ml water were added to the slip. After casting intometal or polymer molds solidification occurred due to the shift of thezeta-potential curve from the alkaline to the acidic pH region caused bythe formation of the deflocculant salicylic acid. The green parts arecharacterized by green densities of 60 to 62%.

EXAMPLE 8

An alkaline casting slip with a pH of 9-10 was prepared by adding 400 galuminum oxide powder (RCHP DBM; Reynolds, USA) to a solution of 0.80 gcitric acid, 5.0 g acetic acid methyl ester and 1.00 g conc. ammoniasolution (25%) in 75.0 g deionized water. After deagglomeration, 80.0 gzirconia powder (TZP, SY Ultra 5.2; ICI Z-Tech, GB) in a mixture of 0.16g citric acid and 0.15 g conc. ammonia solution (25%) in 20.0 gdeionized water (pH 9-10) were added. After further deagglomeration anddegassing, 500 units esterase (EC 3.1.1.1) were added. After castinginto metal or polymer molds, solidification occurred in 15-30 minutes.The green parts were characterized by green densities of 55 to 62%.

EXAMPLE 9

An alkaline casting slip with a pH of 9-9.5 was prepared by adding 350 gzircon oxide (PSZ with 6 mole % Y₂ O₃ Rhone-Poulenc, France) to asolution of 1.75 g acetic acid ethyl ester and 1.4 gammoniumpolyacrylate solution (Darvan C, Wanderbilt, USA) in 49.0 gdeionized water. After deagglomeration and degassing, 500 units esterase(EC 3.1.1.1) were added as a solution with 2000 units/ml to the slip(viscosity ca. 500 mPa s). After casting into metal or polymer molds,solidification occurred due to the shift of the pH from the alkaline tothe acidic region. The green parts were characterized by green densitiesof about 55%.

EXAMPLE 10

An acidic casting slip was prepared by adding 797 g aluminum oxidepowder (A 17 NE, Alcoa, BRD) to a solution of 1.60 g conc. hydrochloricacid and 3.0 g urea in 77.4 g of deionized water. At this time the sliphad a pH of 4.5 to 5.

After addition of 3.0 ml of a binder solution (Acusol 820, Rohm & Haas,Germany or Rohagit SD 15 Rohm, Germany) the suspension had a pH of 4 to4.5 and a viscosity of 100-500 mPa s.

After deagglomeration and degassing 910 units urease (EC 3.5.1.5) wereadded as a solution of 2000 units/ml urease to this processed slip.After casting into metal or polymer molds, solidification occurred after30 to 60 minutes. The coagulated, but not dried green bodies arecharacterized by high coagulation strength. Thus easily demoldability inthe wet form is achieved.

EXAMPLE 11

An alkaline casting slip with a pH of 11 was prepared by adding 673 gsilicon nitride powder (LC 10, Starck, Germany) to a mixture of 40 gLudox TM (Du Pont, USA), 10.0 g Tetramethylammoniumhydroxide solutionand 10.0 g urea in 160.0 g of deionized water. After deagglomeration anddegassing 1500 units Urease powder and 0.20 g glycerine triacetate wereadded to the slurry. After casting into metal or polymer moldssolidification occurred depending upon reaction conditions in 30 to 60minutes. The dried green parts were characterized by green densities of50 to 55%.

EXAMPLE 12

An alkaline casting slip with a pH of 12 to 13 was prepared by adding660 g silicon nitride powder (LC 10, Starck, Germany) to a solution of2.5 g 1,6-Diaminohexane, 6.0 g Tetramthylammoniumhydroxide solution and10.0 g urea in 155 g of deionized water. After deagglomeration anddegassing 500 units Urease powder and 0.20 g glycerine triacetate aswell as 2.0 g of a hydrolysable polymer were added to the slurry. Aftercasting into metal or polymer molds solidification occurred in 30 to 60minutes due to the hydrolysis of the ester group containing polymer anddue to the simultaneously reduction of surface charge by shifting the pHtowards the acidic region.

EXAMPLE 13

An alkaline casting slip with a pH of 11 to 12 was prepared by adding660 g silicon nitride powder (LC 10, Starck, Germany) to a solution of2.5 g 1,6-Diaminohexane, 4.0 g Tetramethylammoniumhydroxide solution and10.0 g urea in 155 g of deionized water. After deagglomeration anddegassing 500 units Urease powder and 0.20 g glycerine triacetate aswell as 2.0 g of a hydrolyzable polymer were added to the slurry.Furthermore 2000 units of an esterase were added. After casting intometal or polymer molds solidification occurred in 30 to 60 minutes dueto the hydrolysis of the ester group containing polymer and due to thesimultaneously reduction of surface charge by shifting the pH towardsthe acidic region.

The previous examples are based on Al₂ O₃, SiC, Si₃ N₄, ZrO₂ and Al₂ O₃/ZrO₂ --composites as ceramic materials. The invention is not limited tothese examples, but is also applicable to all kinds of monolithicceramic materials, such as ZrO₂ (TZP, PSZ, FSZ) Sic, Si₃ N₄, BaTiO₃,TiO₂, mullite, MgO, ball clay, B₄ C, TiB₂, BN and SiO₂. The inventioncan also be used for ceramic composites, e.g. fiber reinforced ceramicssuch as Al₂ O₃ SiC or SiC/C or for particle-reinforced ceramic such asAl₂ O₃ /ZrO₂ and Al₂ O₃ /TiB₂.

Further more, all of the mixtures in examples 1 to 9 are characterizedby the fact that the reactions start in the mixtures at room temperature(15° to 30° C.).

This invention may be embodied in other forms or carried out in otherways without departing from the spirit or essential characteristicsthereof. The present embodiment is therefore to be considered as in allrespects illustrative and not restrictive, the scope of the inventionbeing indicated by the appended claims, and all changes which comewithin the meaning and range of equivalency are intended to be embracedtherein.

What is claimed is:
 1. A process for the production of a ceramic greenpart which comprises:providing a castable, aqueous slip having slipparticles including particles selected from the group consisting ofoxidic particles and non-oxidic particles therein, said slip particleshaving an oxide surface and a surface charge, wherein said surfacecharge is obtained by adding to said slip particles a material selectedfrom the group consisting of an acid and a base; adding an activesubstance to the slip wherein said active substance is a chemical whichis decomposable, forming acidic products, due to time delayed,temperature sensitive reactions; mixing and degassing the resultantslip; casting the mixed and degassed slip into the mold; thermallyactivating the resultant slip and waiting until decomposition productsare formed, said decomposition products changing said surface charge ofthe slip particles leading to solidification, thus forming a wet ceramicgreen part; and demolding the wet ceramic green part.
 2. The process ofclaim 1, wherein the slip has an isoelectric point (IEP) and a pH whichdetermines the solidification, and wherein said pH is shifted from anacidic region to the IEP.
 3. The process of claim 1, wherein the sliphas an isoelectric point (IEP) and a pH which determines thesolidification, and wherein the pH of the slip is shifted from analkaline region to the IEP.
 4. The process of claim 1, wherein a timedelayed self decomposing active substance is added to the slip in theform of organic, nonionic molecules.
 5. The process of claim 1, whereinsaid time delayed, self decomposing active substance is selected fromthe group consisting of glyceroliesters, glyceroltriesters, lactones,and sultones.
 6. The process of claim 1, including the step of adding abiopolymer to the slip, wherein the time delayed self decomposing activesubstances lead to structure changes of the biopolymer and tosolidification of the slip.
 7. The process of claim 6, wherein saidbiopolymer added to the slip is selected from the group consisting ofcarrageenan and alginate.
 8. The process of claim 1, including the stepof adding a colloidal, inorganic sol to the slip, wherein the timedelayed self decomposing active substance leads to additionalcoagulation of the sol particles.
 9. The process of claim 8, whereinsaid colloidal inorganic sol is selected from the group consisting ofsilica, alumina, zirconia and titania particles with a medium particlesize of 5-100 nm and a specific surface area of 20-400 m² /g.